ABSTRACT This work is a continuation of our long-standing efforts to understand the formation of the vertebrate limb. The hope is that the knowledge we glean will ultimately provide insights into the developmental defects leading to congenital malformations and to novel regenerative therapies to restore injured appendages. While replacement of developed limb tissues could, in principle, be achieved by harnessing and enhancing the endogenous regenerative potential of the limb through a process akin to amphibian limb regeneration, an alternative approach is to attempt to recapitulate embryonic limb development in an adult setting. Towards that end, in preliminary studies, we have taken a reprogramming approach. Screening for a cocktail of transcription factors capable of respecifying mesenchymal cells to a limb progenitor identity, we have evidence that Lhx2, Nmyc, Sall4 and PDRM16 together can convert non-limb fibroblasts into cells with the properties of early limb bud cells, including driving expression of a gene profile mimicking that is seen in the early limb bud, and can imbue cells with the potential to differentiate into the range of cell types found in the developing limb. In Aim 1, we will characterize these cells in greater detail in vitro to determine how closely they approximate endogenous limb bud progenitors. In Aim 2, we will interrogate these cells functionally, testing their capacity to form appropriate tissues and structures in vivo. Finally, in Aim 3, we will focus on two of the factors required for putatively reprogramming limb bud progenitor cells, Sall4 and Lhx2, investigating whether they act through the same sets of gene targets during reprogramming as they do during limb bud development. Together, these studies will bring us closer to understanding how to manipulate limb-like cells for regenerative purposes and will also potentially provide new insights into how early limb bud is regulated.